This invention relates in general to an overheat protection system for an electric heating appliance such as an electric blanket and in particular to a temperature protection system which is always effective as well as containing a fail-safe circuit to automatically test the various electrical components and assure that they are functioning properly. In electrical appliances such as electric blankets, it is imperative that protective devices be provided so that the blanket cannot become overheated, causing fire or injury to the user. At one time it had been customary to mount a number of small bimetallic thermostats to sense the temperature at particular locations throughout the blanket. The thermostats were physically spaced along the heating element and electrically connected in series therewith. Such heating elements typically comprised an elongated insulated resistance wire element which was mounted between the layers of the blanket material and disposed in a tortuous configuration to deliver heat uniformly throughout the blanket. If any of the thermostats were subject to an overheat condition the thermostat would open disconnecting power to the electric blanket heating element. However, if an overheat condition existed at a location where no thermostat was mounted, it was possible that such an overheat condition could exist which could not be detected by any thermostat to turn the power to the blanket off.
To correct this situation it then became common to utilize pairs of coaxially wound conductors which formed a part of the heating circuit or a separate sensing circuit. These conductors were insulated from each other under normal temperature operating conditions by a temperature responsive material which would decrease in resistance to establish a lower resistance electrical path between the conductors if an overheat condition occurred. Such an overheat protection system for an electric blanket is disclosed in Crowley U.S. Pat. No. 3,628,093, assigned to the same assignee as the present application.
Blanket control circuits making use of the type of heat sensitive material mentioned above were subsequently developed which completely eliminated the use of any thermostats, thus being termed thermostatless blanket control circuits, and which included relatively inexpensive and uncomplicated thermal overload circuits. One such blanket control circuit is described in the Crowley et al copending U.S. Reissue application Ser. No. 488,213, now U.S. Pat. Re. No. 28,656, also assigned to the same assignee as the present invention. This latter type of blanket control circuit is advantageous in that it replaced the previously used bimetallic thermal switches which were capable of handling only a small amount of power, tended to be overly sensitive and difficult to manufacture and were not as reliable. Although the various circuits used in the thermostatless blankets had advantages over previously used electric blanket control circuits, these thermostatless blanket circuits were not completely fail-safe in that if one or more components failed by an open circuit or short circuit or changed in value, this failure or change in value might cause the control to keep the blanket on at all times even if an overheat condition occurred. Accordingly, it is of great importance to provide some fail-safe device and/or test circuit which would prevent over-heating in the case of a malfunction in the control circuit and make certain that the control circuit components were continuously operating properly.